1. Field of the Invention
This invention relates generally to the detection and/or quantification of water flow, in particularly water flow which is not directly accessible, such as in a pipe, or water flow occurring in a borehole primarily designated to oil production. The present invention can e.g. find application in the production logging techniques directed to the analysis of producing oil wells, or int he injection wells techniques for determining the mechanical integrity of such wells. The invention allows one to provide qualitative and quantitative information related to water flow in a borehole, such e.g. between the casing and the earth formation surrounding the borehole.
2. Description of the Related Art
As known in the art of logging techniques, a well which has been determined to be promising for oil production, is provided with a metallic casing, and cement is injected between the earth formation and the casing. Perforations are then made through the casing/cement and in the oil productive formation, so as to allow oil to flow up to the surface through a tubing beforehand arranged in the well coaxially to the casing. It is of great importance to identify fluid points of entrance to or exit from the borehole, as well as to determine the mechanical integrity of the cement annulus. However, unwanted vertical flow of water can occur in the cement, between the casing and the formation. This phenomenon, usually called "water channeling", causes undesirable paths between formations located at different depths, i.e. at different pressures, e.g. by allowing water from a first formation layer to mix with oil coming from a second formation layer. This phenomenon disturbs substantially the oil production.
The invention may also be used, besides the production wells hereabove referred to, in the so-called "injection wells" which include different categories of wells. In a first category, a high pressure fluid is injected in a first well so as to make the oil move in earth formation towards a neighboring second well which will become the oil producing well. A second category of well is used for disposing of either fluids associated with the production of oil (such as salt water), or liquid hazardous wastes. In both categories, it is important to determine the mechanical integrity of such wells so as to prevent the injected fluid from contaminating undergound sources of drinking water. Such contamination is usually due to some cracks or channels in the cement annulus and/or in the formation close to the well wall. The injection wells are submitted to stringent regulations with this respect. In the United States, for example, the requirement is such that a test of the mechanical integrity of the well, so as to determine the presence of any leak, be done prior to initial injection and further periodically (every five years).
Among the various methods which have been proposed for detecting and characterizing water flow in a well, such as the flow behind casing, is the nuclear method referred to as "oxygen activation", as described e.g. in the article "Advances in nuclear production logging" by P. A. Wichmann et al., Trans., SPWLA (1967). This method is carried out with a tool including a nuclear source emitting high energy neutrons and a gamma ray detector. Oxygen atoms in the water, when interacting with neutrons of sufficient energy, are activated to an unstable state in the form of nitrogen-16 which decays back exponentially in time with a half-life of 7.13 seconds, to stable oxygen-16 while emitting 6.13 Mev and 7.12 Mev gamma rays. This reaction is also referred to as "O.sup.16 (n,p)N.sup.16 " reaction. Count rates in the gamma ray detector are proportional to the total amount of oxygen present around the tool.
An operative oxygen activation logging tool, as described in Louis A. Allaud U.S. Pat. No. 3,603,795 and assigned to Schlumberger Technology Corporation, has been proposed for measuring the speed of water in a polyphase flow, by setting the ratio of the counts referring to the respective near and far detectors.
Some attempts have been made towards a better understanding of the oxygen activation method, see e.g. the article "Quantitative Monitoring of Water Flow Behind and In Wellbore Casing" from D. M. Arnold and H. J. Paap, Journal of Petroleum Engineers of AIME, pages 121-130, Jan. 1979. Based on these experiments, an operative logging tool provided with two detectors has been proposed, as depicted in the article "Measuring Behind Casing Water Flow" by T. M. Williams, presented on May 5-7, 1987 at the International Symposium on Subsurface Injection of Oilfield Brines, in New Orleans. The ratio of the counts from the respective near and far detectors is an exponential function of the velocity of the detected water flow. Some other quantitative information, i.e. the radial distance of the water flow and the volume flow rate are estimated through further calculations.
However, these known methods suffer from major drawbacks. Firstly, they require two detectors, which increases the cost. Secondly, the measurements require long measuring periods in the well, such as several minutes. Thirdly, a prior calibration is needed, even for the mere determination of the velocity of the water flow. In other words, the response of the tool has to be measured when submitted to given external conditions, i.e. to different flow velocities, flow rates, and radial distances of flow, so as to obtain a set of reference signals. These reference measurements, allegedly corresponding to stationary oxygen, are then subtracted from actual measurements. The calibration may be carried out either in a laboratory set-up especially built for that purpose, or in a region of the well assumed to be free of any water flow behind casing but otherwise showing characteristics identical to those of the region of the well under investigation. However, calibration is time consuming and thus costly, and, when carried out in the well, is relatively not reliable since there is no certainty that the region chosen as a reference is actually representative and free of any water leakage. Finally, these known methods are limited with respect to the velocity range of the water flow able to be detected with accuracy, especially in relatively low velocities, below e.g. 3 feet/minute (i.e. 0.016 m/s). This is due in part to the limitations deriving from the calibration process hereabove described. This is all the more detrimental since the velocities of the water flow encountered in a well are often low, i.e. below 3 feet/minute.
Furthermore, the oxygen activation method per se raises another difficulty. The emitted neutrons interact with, and thus activate, all of the oxygen atoms surrounding the tool, i.e. the "flowing oxygen" and the "immobile oxygen". The immobile oxygen is present in the earth formations, in the cement, and in the oxygen bearing fluid which is stationary. The flowing oxygen is the oxygen present in the fluids flowing either downward or upward. The above mentioned known methods attempt to differentiate the flowing oxygen from the immobile oxygen by establishing, through calibration, a set of reference data corresponding to the absence of flow. However, the calibration suffers from severe limitations already hereabove referred to with this respect.
There has been also some attempts to optimize the oxygen activation method by monitoring the tool speed in the well according to the water flow velocity, as e.g. depicted in the article "Examples of Detection of Water Flow by Oxygen Activation on Pulsed Neutron Logs" by W. H. M. DeRosset, in Paper CCC of SPWLA Twenty-seventh annual logging symposium, Jun. 9-13, 1986. However, this method relies on the prior knowledge or estimation of the water flow velocity, and is further sensitive to the spacing between the source and the detectors.
U.S. Pat. No. 4,574,193 shows a method wherein an oxygen activation tool is run twice in the well at two different logging speeds. The water velocity is derived from a displacement distance "D" which reflects the time required for the activated fluid to reach a detector. Nevertheless, the calculation is relatively complicated. Furthermore, this method requires the logging speed be less than the water velocity, which is a drastic limitation since it implies an a priori assumption of the velocity which is the data to be determined.
Many endeavors have been made to improve the oxygen activation method for the detection of water flow, either by combining the oxygen activation method with other logging methods (see e.g. U.S. Pat. Nos. 3,817,328; 4,233,508 and 4,737,636), or with prior knowledge of environmental conditions (see e.g. U.S. Pat. No. 4,032,781), or by designing the detectors with a specific geometry (see e.g. U.S. Pat. No. 4,032,779), or further by setting up a relationship for the ratio of count rates at a detector in two distinct energy regions of the gamma ray spectrum (see e.g. U.S. Pat. No. 4,032,778). All these methods show the drawbacks already referred to hereabove, since they do not depart in principle from the basic oxygen activation method hereabove discussed.
According to the above, there is a need for a reliable method for obtaining quantitative and qualitative information related to water flow, and in a specific application, to water flow behind casing, either for production well analysis or for mechanical integrity test in injection wells.